Confidence is key. And do not mention it is your first OA game.
They will challenge you more, so focus on giving short but loud "good tackle", "all ball", "clear trip" type feedback when giving or not giving a foul. I've found that helps stop the moaning before it starts. Where there is a player who is constantly asking for fouls etc, I tend to speak directly to them (loudly to other can here) saying something along the line "I can referee without your input and shouting for a foul will never make me give one so use your energy in playing the game"